1. Field of the Invention
The present invention relates to coordinate entry devices for computer systems. More particularly, it relates to a floating coordinate system for such a coordinate entry device.
2. History of the Prior Art
Those skilled in the art will readily appreciate that many applications require the entry of location coordinates into a computer. For example, digitizing systems are well known in the art for inputting maps, blueprints or the like into a computer by considering such source documents as consisting of a number of features located at specific coordinates. Thus, after completion of the digitizing process, the computer stores the coordinates of all features on the source document.
In fact, any interactive system, in which data is entered into a computer by indicating, with a special pen, cursor or the like, a point on a board or cathode ray tube screen, necessarily involves the entry of coordinates.
In digitizing systems, features on a source document may be recorded by monitoring the location of a position indicator on a reference surface as it traces the source document features. Components in the position indicator interact with components associated with the reference surface to produce data indicative of the location of the position indicator. Such digitizing systems can be based on a number of different theories. For example, in electrostatic systems, as disclosed in U.S. Pat. Nos. 3,767,858, 3,983,322, 4,022,971 all to Rodgers, an electric field is created about the reference surface by conductors within the reference surface. The phase of the electric field along two coordinate axes varies with position so that the phase of the electric field, as detected by a position indicating device, uniquely determines the position of the device.
In magnetostrictive systems, as disclosed in U.S. Pat. Nos. 3,904,821 and 3,956,588 both to Whetstone et al, 4,018,989 to Snyder et al and 4,081,603 to Davis et al, vibrations are created along two coordinate axes in magnetostrictive material. A position indicator produces a signal when the vibrations are detected by it and the time between initiation and detection of the vibrations represents the location of the position indicator. Obviously, in such systems the vibrations may be created by the position indicator, and vibration sensors disposed at locations fixed with respect to the reference surface may be employed to determine the location of the position indicator.
Finally, electromagnetic systems, as disclosed in U.S. Pat. Nos. 3,904,822 to Kamm et al, 3,873,770 to Ioannou, 3,735,044 to Centner et al, 3,700,809 to Nadon, and 3,647,963, 3,725,760 and 3,801,733 all to Bailey, sense an electromagnetic field, created by either elements associated with the reference surface or elements associated with the position indicating device as an indication of position. The devices disclosed in the Bailey patents employ plural grid elements in the reference surface. The position indicator includes a coil which generates a magnetic field about the two sets of conductors. The ratio of the amplitudes of the signals induced in the two sets of conductors is indicative of relative position. Alternatively, the patents to Ioannou and Kamm disclose means for causing a magnetic field to sweep across a reference surface. A coil in the position indicating device detects the passage of the magnetic field. The period of time between the initiation of the magnetic field sweep and its detection by the position indicating device is related to the location of the position indicator transverse to the direction of the magnetic field.
The position indicating devices in these systems can be of several forms. In one form, the device is constructed with a flat bottom surface which is adapted to rest on the reference surface. Other position indicating devices are hand-held and include a body which is similar to a pen or pencil. All such position indicating devices shall be referred to hereinafter generically as "cursors". If the cursor is free to move away from the reference surface, (i.e., it is not mechanically connected to the reference surface) it will be referred to herein as a "free cursor".
No matter what type of system is employed to determine the position of a cursor on a reference surface in whatever application, it is often necessary to determine the position of a cursor with respect to more than one coordinate system. For example, in digitizing systems as described above, a "menu" is employed to select computer commands. For example, if an operator desires to store a circle at a particular location, rather than drawing or tracing a circle freehand, the circle command might be selected from the menu. Then the operator locates a first point on the reference surface indicative of the position of the center of the circle and a second point on the reference surface indicative of the radius of the circle. The computer then automatically stores a circle at the identified position.
In other applications, an operator might wish to associate a point on one source document with a point on a second source document. Thus, in a sense, the operator is associating a point in one coordinate system with a point in another coordinate system. In many other applications, it is necessary for an operator to constantly switch between working in one coordinate system and working in another coordinate system.
This switching has been accomplished in the prior art in several different manners. For example, one source document which is related to a first coordinate system can be removed from a reference surface and replaced by a second source document which is related to a second coordinate system. Then, instructions must be supplied to the computer that the source document and the coordinate system have been changed. If an operator must frequently switch between source documents, the operator's task is slow, tedious and inefficient.
Alternatively, systems have been developed with two independent reference surfaces with either one or two cursors. In such systems, an operator simply switches from board to board to switch from one coordinate system to another. Although this certainly speeds operations, drawbacks still exist. Specifically, the operator must still turn from one board to another which is both time consuming and exhaustive. If one of the reference surfaces is large and the other small, one reference surface may be positioned on top of the other reference surface. However, to obtain access to the lower reference surface, it will often be necessary to move the smaller reference surface. As can be imagined from the discussion hereinabove, the structure which must be associated with a reference surface to create or detect signals causes the reference surface to be quite heavy. Thus, a day's worth of moving the smaller reference surface about the larger reference surface proves to be a clumsy, tedious and exhausting task.
Additionally, such two reference surface systems tend to be wasteful, since supplying completely independent signal generating or sensing means in each of the two reference surfaces is duplicative. In precision digitizing equipment, such reference surfaces are made with a high degree of precision and are therefore expensive. Having two independent reference surfaces needlessly increases the cost of such systems.